The present invention relates generally to scintillators for absorbing and concentrating radioactive ions within a solution, and light sources formed from the scintillators. The invention particularly relates to modified solid scintillators having an ionic surface layer which absorbs radioactive ions.
Contaminated surface and ground water and effluents from power generating stations and industrial sites often contain radioactive particles. These solutions are continuously or intermittently monitored to determine the level of radioactive particles in solution. A substantial amount of the radiation in these solutions is present as positive ions, such as uranium, cesium and actinides, and negative ions, such as iodate and TcO.sub.4.sup.-. These ions produce gamma-rays, x-rays and other electromagnetic radiation which is detected using conventional external detectors including Geiger counters and gas proportional detectors. Electromagnetic radiation can be detected by external sources because it has a relatively long penetration length through a surrounding medium. .alpha. and .beta. particles, however, have a short penetration length through the surrounding medium, preventing these particles from being detected by an external detector.
Radiation emitted by radioactive ions within a solution has been counted using liquid scintillation techniques. A sample of solution is prepared for liquid scintillation analysis by mixing the sample with an aqueous compatible liquid scintillation cocktail or by extracting the radioisotopes from the sample and adding an organic solvent. During liquid scintillation counting, electromagnetic or particulate radiation emitted by radioactive ions impinges on or passes near fluorescent dye molecules (fluors) capable of emitting light in response to the radiation. Energy emitted by the radiation excites the fluor and the fluor emits light. This light emission is known as scintillation. These optical events are detected by a sensitive detection device, such as a photomultiplier tube, or a charge coupled device and are converted into corresponding electrical pulses.
The radioactive content of samples containing an appreciable amount of suspended organic or particulate matter cannot be reliably determined via liquid scintillation counting. The suspended matter within the sample prevents the light emitted from a fluor from being optically detected. Although a sample can be filtered to remove the suspended matter, radioactive ions that are bound to the suspended matter are also removed during filtration, reducing the radioactivity count for the sample.
Radiation has also been detected by adding a solid scintillator to a sample solution. The solid scintillator is composed of a fluor that is typically dispersed in a plastic medium, such as polystyrene as described by Bross et al., Nucl. Instr. and Meth. A307, pp. 35-46 (1991), D'Ambrosio et al., Nucl. Instr. and Meth. A307, pp. 430-435 (1991), Majewski et al., Nucl. Instr. and Meth. A281, pp. 500-507 (1989), Zorn et al., Nucl. Instr. and Meth. A273, pp. 108-116 (1988), and Zorn et al., Nucl. Instr. and Meth. A271, pp. 701-703 (1988). Radiation emitted by radioactive ions within the solution that are in close proximity to the solid scintillator will excite the fluor and produce detectable optical events. Radioactive ions dispersed throughout the remainder of the solution will emit .alpha. and .beta. particles that are too far removed from the scintillator to penetrate the scintillator and excite the fluor.
A scintillator including an ionic layer separated from a solid scintillator by supporting layers has been described. Inzelt et al., J. Electroanal. Chem. 230, pp. 257-265 (1987) disclose an ionic polymeric layer and an associated support structure on a glass scintillator. Their method involves evaporating a metal film onto a polymer film to form a support structure. The ionic polymeric layer is then applied on top of the metal film. The polymer film that supports the metal film and the ionic polymeric layer is placed on a glass scintillator plate. The ionic polymeric layer absorbs radioactive ions from solution. A disadvantage associated with thins scintillator is that radioactive decay particles must penetrate the metal and polymer support layers in order to penetrate the scintillator. Weakly penetrating particles that cannot penetrate the support layers will not be detected.
There is a need for a scintillator that can absorb radioactive ions within a solution and enable direct detection of particulate and electromagnetic radiation regardless of the turbidity of the sample solution. A light source for providing light over a uniform area is also needed.